Minor: add a simple List<T>.indexOfOrThrow() utility and use it in a new LedgerTransaction.outRef helper.

core/src/main/kotlin/core/Transactions.kt

@@ -85,8 +85,15 @@ data class WireTransaction(val inputs: List<StateRef>,
         return SignedTransaction(serialized, withSigs)
     }
 
+    /** Returns a [StateAndRef] for the given output index. */
     @Suppress("UNCHECKED_CAST")
-    fun <T : ContractState> outRef(index: Int) = StateAndRef(outputs[index] as T, StateRef(id, index))
+    fun <T : ContractState> outRef(index: Int): StateAndRef<T> {
+        require(index >= 0 && index < outputs.size)
+        return StateAndRef(outputs[index] as T, StateRef(id, index))
+    }
+
+    /** Returns a [StateAndRef] for the requested output state, or throws [IllegalArgumentException] if not found. */
+    fun <T : ContractState> outRef(state: ContractState): StateAndRef<T> = outRef(outputs.indexOfOrThrow(state))
 
     override fun toString(): String {
         val buf = StringBuilder()

core/src/main/kotlin/core/Utils.kt

@@ -64,6 +64,13 @@ fun <T> SettableFuture<T>.setFrom(logger: Logger? = null, block: () -> T): Setta
 // Simple infix function to add back null safety that the JDK lacks:  timeA until timeB
 infix fun Temporal.until(endExclusive: Temporal) = Duration.between(this, endExclusive)
 
+/** Returns the index of the given item or throws [IllegalArgumentException] if not found. */
+fun <T> List<T>.indexOfOrThrow(item: T): Int {
+    val i = indexOf(item)
+    require(i != -1)
+    return i
+}
+
 // An alias that can sometimes make code clearer to read.
 val RunOnCallerThread = MoreExecutors.directExecutor()